Hydrogen is expected as a future energy medium, and accordingly active researches and developments are being performed on wide technical fields including the production, storage and transportation, and utilization of hydrogen. The advantages provided by the use of hydrogen as an energy medium include high energy utilization efficiency, and additionally the fact that the emission after combustion is limited to water.
As matters stand, fossil fuels such as petroleum, coal and natural gas account for about 80% of the primary energy; in the future the proportion of the fossil fuels will be gradually decreased due to the increasing use of renewable energy, but, even so, it is anticipated that the proportion of the fossil fuels will remain at a high level. Accordingly, it can be said that in the production of hydrogen, the importance of the route in which the fossil fuels are used as the raw materials for the source of the primary energy is not diminished for the time being.
However, when hydrogen is produced by using a carbon-containing fuel such as a fossil fuel, CO2 is emitted.
For the purpose of preventing the global warming, the reduction of the CO2 emission is said to be an urgent and important issue. Under such circumstances, a technique for separating and recovering the CO2 by-produced when hydrogen is produced from a fossil fuel is important as a technique capable of coping with both the reduction of the CO2 emission and the early realization of the hydrogen-dependent society.
There have hitherto been known the methods for separating CO2 when hydrogen is produced by using as a raw material a carbon-containing fuel such as a fossil fuel.
As a first method of such methods, a method may be quoted in which high-purity hydrogen is obtained with a pressure swing adsorption (PSA) apparatus from a mixture composed of hydrogen, CO, CO2 and methane produced by using as a raw material a fossil fuel through steam reforming and shift reaction, and on the other hand, CO2 is highly purified, and separated and recovered with a chemical absorption method from the impurity-containing offgas. However, it can not be said that this method is excellent from the viewpoint of the energy efficiency of the hydrogen production because of the reasons including the fact that in the chemical absorption method that uses as the absorption solution an organic amine solution, a large amount of energy is required in the step of recovering CO2, namely, the step of regenerating the absorption solution.
As a second method, Patent Document 1 discloses a method in which in a process for purifying and separating individual gases with a two-stage purification apparatus, a CO2 concentrating apparatus is disposed in the first stage to yield a gas flow mainly composed of CO2, a PSA apparatus disposed in the second stage treats the gas discharged with a reduced CO2 concentration from the CO2 concentrating apparatus to yield high-purity hydrogen, and at the same time, CO2 is liquefied and separated from the CO2-enriched gas streams obtained from the respective stages. However, the energy efficiency of this method tends to be low because for the purpose of removing CO2 from the gas, before the separation of hydrogen, containing hydrogen in a large amount, it is inevitably required to adopt a method in which the CO2 recovery apparatus has an extremely high selectivity and is generally large in energy consumption.
As a third method, Patent Documents 2 and 3 disclose a process in which high-purity hydrogen is produced with a hydrogen purification apparatus such as a PSA apparatus, and at the same time, the offgas containing CO2, hydrogen and the like is combusted and thereafter CO2 is recovered. However, in this method, the offgas from the hydrogen purification apparatus, which gas still contains hydrogen in a large amount, is combusted to thereby reduce the hydrogen yield. Further, when air is used in the combustion, the load on the CO2 recovery is large, because nitrogen is present. Alternatively, pure oxygen can be used for combustion; however, in this case, a large amount of energy is consumed for the production of pure oxygen and hence the energy efficiency tends to be low.
As a fourth method, Patent Document 4 discloses a method that uses a PSA apparatus having a serial configuration composed of two or more adsorption towers different from each other in adsorption selectivity. However, the absorption-regeneration cycle of this system is extremely complicated, and hence, it is difficult for this method to suppress the increase of the system cost.
As a fifth method, Patent Documents 5 and 6 disclose a method in which hydrogen is separated from the PSA offgas by using a membrane and the thus separated hydrogen is reused by recycling the thus separated hydrogen to the inlet of the PSA apparatus. According to these documents, however, CO2 is discarded without being recovered, and no description on the treatment method of CO2 such as a concentrating method is found in these documents.
Patent Document 1: Japanese National Publication of International Patent Applications No. 2004-519538
Patent Document 2: Japanese Patent Laid-Open No. 2004-292240
Patent Document 3: Japanese Patent Laid-Open No. 2003-81605
Patent Document 4: U.S. Pat. No. 4,913,709
Patent Document 5: U.S. Pat. No. 4,229,188
Patent Document 6: U.S. Pat. No. 5,435,836